Binding raw devices in Linux kernel 2.6

[galapogos]

Hi,

I'm trying to port an old program compiled on kernel 2.4, onto kernel
2.6. I've using a Mandriva 2009.1 box as my development PC. I've
managed to get the program to build properly once I've gotten the
necessary libraries, and it also runs.

However, one of the first things that the program does is to bind the
raw devices(already set up as separate logical partitions within the
drive as /dev/sdaN) to /dev/rawN, in order to access them later
directly.

To do so, the first thing that it does is to open /dev/rawctl with the
following code:

[code]
if ((master_fd = open("/dev/rawctl", O_RDWR, 0)) < 0) {
return ERR_FAILURE;
}
[/code]
However, open always returns -1, hence the program fails at this
point.

Has anything changed between kernel 2.4 and 2.6 that may cause a
failure when I try to open /dev/rawctl?

Thanks!

[David W. Hodgins]

On Fri, 03 Jul 2009 04:40:07 -0400, galapogos <goi...@gmail.com> wrote:

> [code]
> if ((master_fd = open("/dev/rawctl", O_RDWR, 0)) < 0) {
> return ERR_FAILURE;

> Has anything changed between kernel 2.4 and 2.6 that may cause a
> failure when I try to open /dev/rawctl?

From "man raw" ...

WARNING
The rawio is a deprecated interface since Linux kernel 2.6.3. Please, modify your application to open the
block device with the O_DIRECT flag.

Regards, Dave Hodgins

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[Jean-David Beyer]

David W. Hodgins wrote:
> On Fri, 03 Jul 2009 04:40:07 -0400, galapogos <goi...@gmail.com> wrote:
>
>> [code]
>> if ((master_fd = open("/dev/rawctl", O_RDWR, 0)) < 0) {
>> return ERR_FAILURE;
>> Has anything changed between kernel 2.4 and 2.6 that may cause a
>> failure when I try to open /dev/rawctl?
>
> From "man raw" ...
>
> WARNING
> The rawio is a deprecated interface since Linux kernel 2.6.3. Please, modify your application to open the
> block device with the O_DIRECT flag.
>
> Regards, Dave Hodgins
>

Yes, it is sad, but if you have proprietary software (I was running IBM's
DB2 dbms), you are out of luck. You may have to pay the vendor to upgrade.
If the vendor is in business, that is.

What I did was switch from DB2 to postgreSQL. You may not have an option
like that.

--
.~. Jean-David Beyer Registered Linux User 85642.
/V\ PGP-Key: 9A2FC99A Registered Machine 241939.
/( )\ Shrewsbury, New Jersey http://counter.li.org
^^-^^ 10:10:01 up 14 days, 20:59, 3 users, load average: 4.16, 4.17, 4.13

[Jan Kandziora]

galapogos schrieb:

>
> However, one of the first things that the program does is to bind the
> raw devices(already set up as separate logical partitions within the
> drive as /dev/sdaN) to /dev/rawN, in order to access them later
> directly.
>

Forgetting about "raw devices" and using O_DIRECT should do the trick. But
first read Linus' ramblings about O_DIRECT and "raw io" in general:

http://kerneltrap.org/node/7563

Kind regards

Jan

[galapogos]

Thanks. Does that mean that there's no way of using binding devices
to /dev/rawN? The original code author has left and I'm not sure why
raw devices were used rather than using ioctl calls.

So, by O_DIRECT, you do mean sending ioctl() to the device with
O_DIRECT flag, right?

[Jan Kandziora]

galapogos schrieb:

>
> Thanks. Does that mean that there's no way of using binding devices
> to /dev/rawN? The original code author has left and I'm not sure why
> raw devices were used rather than using ioctl calls.
>

You could rely on this deprecated kernel feature and activate it in the
kernel of your machine, but as you said, you'll try to *port* the software
to Linux-2.6, it seems pretty straightforward to leave out raw devices
entirely.

> So, by O_DIRECT, you do mean sending ioctl() to the device with
> O_DIRECT flag, right?
>

No. O_DIRECT is applied on open(). You don't have to "bind" a raw
device /dev/raw/raw3 to a block device like /dev/sda6 using /dev/rawctl or
the "raw" utility, then use /dev/raw/raw3 instead of /dev/sda6.

Just open /dev/sda6 with O_DIRECT flag. You can even open simple files with
O_DIRECT. Make sure to

#define _GNU_SOURCE

before any #include. BUT, if you plan to really rework your application, it
may be wise to take a side look to madvise() and posix_fadvise().

Kind regards

Jan

[galapogos]

Thanks. It turned out I didn't enable the deprecated raw. It works
after I do so. If I run into any other problems I'll check out the
O_DIRECT. However, conservations with the original author of the code
revealed that the motivation to use raw instead of O_DIRECT was to
bypass any caching that the motherboard chipset does so that the write
happens immediately. This is important since we're dealing with timing
critical events. Apparently O_DIRECT isn't as "direct" in his
experience.